Digital audio amplifier including phase lead-lag compensator for increasing self-oscillation frequency

ABSTRACT

Provided is a digital audio amplifier capable of increasing self-oscillation frequency by using a phase lead-lag compensator. The digital audio amplifier includes the phase lead-lag compensator which increases the self-oscillation frequency by lead-lag-compensating for the phase of an output signal and making a feedback of the compensated output signal. In addition, the digital audio amplifier further includes a bandwidth control means which controls the bandwidth of an error amplifier. Accordingly, the digital audio amplifier can adjust the self-oscillation frequency and reduce the extent to which the self-oscillation frequency varies in accordance with the variation of the output signal by using the bandwidth control means that inserts a pole into the error amplifier.

FIELD OF THE INVENTION

The present invention relates to a self oscillation-type digital audioamplifier, and more particularly, to a digital audio amplifier capableof increasing self-oscillation frequency by using a phase lead-lagcompensator.

BACKGROUND OF THE INVENTION

FIG. 1 is a circuit diagram of a conventional self oscillation-typedigital audio amplifier. Referring to FIG. 1, the conventional selfoscillation-type digital audio amplifier includes a power PMOStransistor (PM), a power NMOS transistor (NM), a first filter 11, asecond filter 12, a voltage divider 13, an error amplifier 14, acomparator 15, and a gate driver 16.

The operation of the conventional self oscillation-type digital audioamplifier of FIG. 1 will be described in the following paragraphs.

An output voltage V_(OUT) is divided by the voltage divider 13 at aconstant rate, and a result of the division, i.e., a feedback voltageV_(FB), is compared with an input audio voltage V_(IN) by the erroramplifier 14. Error amplified by the error amplifier 14 is convertedinto a pulse signal by the comparator 15 having hysteresis. The pulsesignal is transmitted to the power transistors PM and NM via the gatedriver 16 and then is finally used for controlling the output voltageV_(OUT).

More specifically, if the input audio voltage V_(IN) is larger than thefeedback voltage V_(FB), the PMOS transistor PM is turned on so that theoutput voltage V_(OUT) increases. If the output voltage V_(OUT) keepsincreasing until the feedback V_(FB) becomes larger than the input audiovoltage V_(IN), the NMOS transistor NM is turned on so that the outputvoltage V_(OUT) decreases.

The conventional self oscillation-type digital audio amplifier operatesin an oscillation manner where the output voltage V_(OUT) alternatelyincreases and decreases at high speed centering around a result ofamplifying the input voltage V_(IN) at a predetermined rate. The outputvoltage V_(OUT) has a waveform obtained by adding a voltage ripplegenerated in self-oscillation to a result of amplifying the input signalVIN having a voice bandwidth of 20 Hz˜20 kHz at a predetermined rate.Here, the voltage ripple is a voltage component having an amplitude ofabout 100 mV and having a higher frequency than voice signals. In themeantime, since the power switches, i.e., the power transistors PM andNM, automatically operate while the conventional self oscillation-typedigital audio amplifier undergoes self-oscillation, self-oscillationfrequency can be simply called switching frequency.

In the conventional self oscillation-type digital audio amplifier ofFIG. 1, a resistor R₁ and a capacitor C₁ in the first filter 11 areconnected to each other in cascade in order to sense the variation ofthe output voltage V_(OUT). Accordingly, the variation of voltage at thecapacitor C₁ is proportional to the amount of current passing throughthe capacitor C₁. A difference between voltages respectively at eitherend of the resistor R₁ corresponds to the variation of the voltage atthe capacitor C₁. Therefore, as large a waveform as the variation of thevoltage at the capacitor C₁ can be obtained by increasing the resistanceof the resistor R₁.

The conventional self oscillation-type digital audio amplifier can feedback the output voltage V_(OUT) and its variation by sensing thevariation of the voltage at the capacitor C₁ taking advantage of theabove-described structure of the first filter 11 in which the resistorR₁ and the capacitor C₁ are connected to each other in cascade. However,a voltage ripple at the output node (V_(OUT)) of the capacitor C₁, mayundesirably increase. In order to prevent this problem, excessivevoltage ripples should be compensated for by connecting the first filter11 and the second filter 12 in cascade.

Alternatively, voltage ripples at the output node (V_(OUT)) can besufficiently compensated for by providing the first filter 11 withoutthe resistor R1. In this case, however, self-oscillation frequency maydramatically decrease.

SUMMARY OF THE INVENTION

The present invention provides a self oscillation-type digital audioamplifier which is capable of increasing self-oscillation frequency bylead-lag-compensating for the phase of an output signal and making afeedback of the compensated output signal and is also capable ofadjusting the self-oscillation frequency and decreasing the extent towhich the self-oscillation frequency varies in accordance with thevariation of the output signal by controlling the bandwidth of an erroramplifier.

According to an aspect of the present invention, there is provided adigital audio amplifier. The digital audio amplifier includes a powerPMOS transistor, which has a source to which a first power supplyvoltage is applied and a drain connected to a common node, a power NMOStransistor, which has a drain connected to the common node and a sourceto which a second power supply voltage is applied, an output nodefilter, which is connected to the common node, a phase lead-lagcompensator, which lead-lag compensates for the phase of an outputsignal of the output node filter, an error amplifier, which compares anoutput signal of the phase lead-lag compensator with an input audiosignal and amplifies an error between the output signal and the inputaudio signal, a comparator, which converts an error amplified by theerror amplifier into a pulse signal, and a gate driver, which controls agate of the power PMOS transistor and a gate of the power NMOStransistor in response to the pulse signal. Here, the phase lead-lagcompensator increases self-oscillation frequency bylead-lag-compensating for the phase of the output signal of the outputnode filter.

Preferably, the digital audio amplifier further includes a bandwidthcontrol means, which adjusts the bandwidth of the error amplifier.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages of the present inventionwill become more apparent by describing in detail exemplary embodimentsthereof with reference to the attached drawings in which:

FIG. 1 is a circuit diagram of a conventional self oscillation-typedigital audio amplifier;

FIG. 2 is a circuit diagram of a self oscillation-type digital audioamplifier according to a preferred embodiment of the present invention;

FIG. 3 is a diagram illustrating the variation of gain and phase delayin accordance with frequency of a phase lead-lag compensator of FIG. 2;

FIG. 4 is a diagram illustrating limiting of amplification frequencybandwidth by using a bandwidth control means that inserts a pole into anerror amplifier of FIG. 2;

FIG. 5 is a diagram illustrating the frequency characteristics of anentire feedback loop in the self oscillation-type digital audioamplifier of FIG. 2;

FIG. 6 is a circuit diagram of a self oscillation-type digital audioamplifier according to a second embodiment of the present invention; and

FIGS. 7 through 9 are circuit diagrams of different examples of thephase lead-lag compensator of FIG. 2 or 6.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the present invention will be described more fully withreference to the accompanying drawings in which preferred embodiments ofthe invention are shown.

FIG. 2 is a circuit diagram of a self oscillation-type digital audioamplifier according to a preferred embodiment of the present invention.Referring to FIG. 2, the self oscillation-type digital audio amplifierincludes a power PMOS transistor PM1, a power NMOS transistor NM1, anoutput node filter 21, a phase lead-lag compensator 22, an erroramplifier 24, a comparator 25, and a gate driver 26. A speaker 27 isconnected to the output node filter 21.

A first power supply voltage V_(DD), i.e., a positive power supplyvoltage, is applied to the source of the power PMOS transistor PM1, anda common node V_(D) is connected to the drain of the power PMOStransistor PM1. On the other hand, the common node V_(D) is connected tothe drain of the power NMOS transistor NM1, and a second power supplyvoltage Vss, i.e., a negative power supply voltage, is applied to thesource of the power NMOS transistor NM1.

The output node filter 21 is connected to the common node V_(D), and afmal output signal V_(OUT) of the self oscillation-type digital audioamplifier is output from the output node filter 21. The output nodefilter 21 includes an inductor L provided between the common node V_(D)and an output node V_(OUT) of the output node filter 21 and a capacitorC provided between the output node V_(OUT) of the output node filter 21and ground voltage GND.

The phase lead-lag compensator 22 lead-lag compensates the phase of theoutput signal V_(OUT) received from the output node filter 21 and thenoutputs a result of the lead-lag compensation. The phase lead-lagcompensator 22 includes a first resistor R₁ provided between the outputnode of the output node filter 21 and the output node of the phaselead-lag compensator 22, a capacitor C₁ connected to the first resistorR₁ in parallel, and a second resistor R₂ provided between the an outputnode of the phase lead-lag compensator 22 and the ground voltage GND.

The error amplifier 24 compares an output signal V_(FB) of the phaselead-lag compensator 22 with an input audio signal V_(IN) and amplifiesan error between the output signal V_(FB) and the input audio signalV_(IN). The comparator 25 converts the amplified error received from theerror amplifier 24 into a pulse signal, and the gate driver 26 controlsthe gate of the power PMOS transistor PM1 and the gate of the power NMOStransistor NM1 in response to the pulse signal.

The error amplifier 24 includes an operational amplifier whose outputnode is connected to an input node of the comparator 25 and whose firstinput node is connected to the input audio signal V_(IN), a thirdresistor R₃ provided between an output node V_(FB) of the phase lead-lagcompensator 22 and a second input node of the operational amplifier 23,a fourth resistor R₄ provided between the second input node of theoperational amplifier 23 and an output node V_(OP), and a capacitor C₄connected to the fourth resistor R₄ in parallel. Functions of the erroramplifier 24 whose bandwidth is limited are determined by the capacitorC₄. In other words, a pole is generated at a predetermined frequency atwhich the capacitor C₄ and the fourth resistor R₄ have the sameimpedance. Hereinafter, the operation of the self oscillation-typedigital audio amplifier according to the present invention, shown inFIG. 2, will be described in greater detail. In principle, an oscillatoroscillates at a predetermined frequency that provides a phase delay of atotal of 360 degrees and a gain of 1 or larger in an entire feedbackloop. The self-oscillation frequency can be increased by increasing thephase margin of the feedback loop. From a phase's point of view, theconventional self oscillation-type digital audio amplifier of FIG. 1increases its phase margin by inserting the cascade resistor R₁ into thecapacitor C₁, while the self oscillation-type digital audio amplifieraccording to the present invention increases its phase margin by usingthe phase lead-lag compensator 22.

FIG. 3 is a diagram illustrating the variation of gain and phase delaydepending on frequency of the phase lead-lag compensator 22 of FIG. 2.The phase lead-lag compensator 22 can compensate for phase delaygenerated at the output node filter 21. According to the frequencycharacteristics of the phase lead-lag compensator 22, a zero point iscreated by the first resistor R₁ and the capacitor C₁, and gainincreases. If the frequency of the phase lead-lag compensator 22increases during a phase lead, the gain stops increasing due to a polecreated by the capacitor C₁ and the first and second resistors R₁ and R₂connected to each other in parallel, and the phase lead decreases. Then,the original phase is recovered at a frequency 10 times higher than thepole.

Therefore, phase margin can be generated at frequencies between the zeropoint and the pole by leading the output node filter 21. The phasemargin indicates that phase delay caused by the error amplifier 24 andthe comparator 25 may possibly increase. The phase margin increasesself-oscillation frequency.

FIG. 4 is a diagram illustrating limiting of amplification frequencybandwidth by using the bandwidth control means C₄ that inserts a poleinto the error amplifier 24. The error amplifier 24 into which a pole isinserted maintain uniform gain at frequencies lower than the pole buthas decreasing gain and lengthened phase delay at frequencies higherthan the pole. If the pole moves within the phase variation bandwidth ofthe phase lead-lag compensator 22, phase delay of the entire feedbackloop varies at self-oscillation frequency or so, and accordingly, theself-oscillation frequency itself varies. The error amplifier 24 intowhich the pole is inserted may possibly be seen as an amplifier whoseoperation bandwidth is limited.

FIG. 5 is a diagram illustrating the frequency characteristics of anentire feedback loop in a self oscillation-type digital audio amplifieraccording to a preferred embodiment of the present invention. A low passfilter, i.e., the output node filter 21 of FIG. 2, which is an secondaryinductor-capacitor filter, removes self-oscillation frequency componentswhose frequency is higher than that of the input audio signal V_(IN)from a signal output from the common node VD of the power MOStransistors PM1 and NM1. In the output node filter 21, the gain of avoltage component whose frequency is higher than self-oscillationfrequency (ƒ) decreases to 40 dB/dec, and the phase of the voltagecomponent is delayed by as much as 180 degrees. Here,

$f = {\frac{1}{2\;\pi\;\sqrt{LC}} \cong {20\mspace{14mu}{{kHz}.}}}$However, if the frequency of the voltage component further increases, azero point is created due to parasitic resistance existing in thecapacitor C, and a phase lead occurs. Since it is generally generated ata frequency of several MHz or higher, the zero point is almost ignored.

The output signal V_(OUT) of the output node filter 21 is converted intothe feedback signal V_(FB) after its amplitude is decreased and itsphase is led by the phase lead-lag compensator 22. The error amplifier24 compares the feedback signal V_(FB) with the input audio signalV_(IN), input from the outside of the self oscillation-type digitalaudio amplifier of FIG. 2, and amplifies a difference between thefeedback signal V_(FB) and the input audio signal V_(IN).

While signals travel along the feedback loop in the selfoscillation-type digital audio amplifier of FIG. 2, phase delayadditionally occurs in the comparator 25 and the gate driver 26. In thepower MOS transistors PM1 and NM1, in particular, phase inversionoccurs, and accordingly, a phase delay of 180 degrees occurs. Finally,the self oscillation-type digital audio amplifier of FIG. 2 startsoscillating at a predetermined frequency where a total phase delay of360 degrees can be obtained with a loop gain of 1 or larger. Gain andphase delay can be adjusted by using the phase lead-lag compensator 22and the error amplifier. If a self oscillation-type digital audioamplifier includes the output node filter 21 without the phase lead-lagcompensator 22, self-oscillation frequency remains at several tens ofkHz. On the other hand, if the self oscillation-type digital audioamplifier, like the one shown in FIG. 2, includes the phase lead-lagcompensator 22 as well as the output node filter 21, theself-oscillation frequency increases to several hundreds of kHz.

In FIGS. 3 through 5, frequency (W) and gain (Gain) are representedusing a log scale.

In a self oscillation-type digital audio amplifier, self-oscillationfrequency decreases as output voltage increases. If bandwidth is limitedby inserting a pole into the error amplifier 24 using the bandwidthcontrol means C₄, the extent to which the self-oscillation frequencyvaries in accordance with the variation of the output voltage can bereduced. In other words, if the self-oscillation frequency decreases,the error amplifier 24 increases gain and decreases phase delay. On theother hand, if the self-oscillation frequency increases, the erroramplifier 24 decreases the gain and increases the phase delay. By dongso, the error amplifier 24 can prevent fluctuations in theself-oscillation frequency in accordance with the variation of theoutput voltage.

FIG. 6 is a circuit diagram of a self oscillation-type digital audioamplifier according to another preferred embodiment of the presentinvention. Referring to FIG. 6, the self oscillation-type digital audioamplifier, like the self oscillation-type digital audio amplifier ofFIG. 2, includes a power PMOS transistor PM1, a power NMOS transistorNM1, a phase lead-lag compensator 22, an error amplifier 24, acomparator 25, a gate driver 26, and a speaker 27. In addition, the selfoscillation-type digital audio amplifier further includes an output nodefilter 61 in which, unlike in the output node filter 21 of FIG. 2, aresistor R is provided between a capacitor C and ground voltage GND.

In the present embodiment, self-oscillation frequency of an entirefeedback loop increases due to the existence of the resistor R in theoutput node filter 61. More specifically, due to the resistor Rconnected to the capacitor C in cascade, a zero point created due toparasitic resistance ESR and the capacitor C moves down so that phasedelay occurring in the output node filter 61 decreases and phase marginsin the error amplifier 24, the comparator 25, and the gate driver 26increase. Accordingly, the self oscillation-type digital audio amplifiercan oscillate at higher frequency.

FIGS. 7 through 9 are diagrams illustrating different examples of thephase lead-lag compensator 22 of FIG. 2 or 6.

Referring to FIG. 7, a phase lead-lag compensator includes a resistor R₁provided between an output node V_(OUT) of an output node filter and aninner node N, a capacitor C₁ connected to the resistor R₁ in parallelbetween the output node V_(OUT) and the inner node N, a resistor R₂provided between an output node V_(FB) of the phase lead-lag compensatorand the inner node N, and a resistor R₃ provided between the output nodeV_(FB) of the phase lead-lag compensator and ground voltage GND.

Referring to FIG. 8, a phase lead-lag compensator includes a resistor R₁provided between an output node V_(OUT) of an output node filter and anoutput node V_(FB) of the phase lead-lag compensator, a resistor R₂provided between the output node V_(FB) of the phase lead-lagcompensator and an inner node N, a resistor R₃ provided between theinner node N and ground voltage GND, and a capacitor C₁ provided betweenthe output node V_(OUT) of the output node filter and the inner node N.

Referring to FIG. 9, a phase lead-lag compensator includes a resistor R₁provided between an output node V_(OUT) of an output node filter 62 andan output node V_(FB) of the phase lead-lag compensator, a capacitor C₁connected to the resistor R₁ in parallel between the output node V_(OUT)of the output node filter 62 and the output node V_(FB) of the phaselead-lag compensator, a resistor R₂ provided between the output nodeV_(FB) of the phase lead-lag compensator and ground voltage GND, and acapacitor C_(b) provided between the output node V_(FB) of the phaselead-lag compensator and a connection node N between a capacitor C and aresistor R in the output node filter 62.

While the present invention has been particularly shown and describedwith reference to exemplary embodiments thereof, it will be understoodby those of ordinary skill in the art that various changes in form anddetails may be made therein without departing from the spirit and scopeof the present invention as defined by the following claims.

As described above, the self oscillation-type digital audioamplifieraccording to the present invention is capable of increasingself-oscillation frequency by lead-lag-compensating for the phase of anoutput signal and making a feedback of the compensated output signal,and is also capable of adjusting the self-oscillation frequency andreducing the extent to which the self-oscillation frequency varies inaccordance with the variation of the output signal by controlling thebandwidth of an error amplifier, i.e., by inserting a pole into theerror amplifier.

1. A digital audio amplifier comprising: a power PMOS transistor whichhas a source to which a first power supply voltage is applied and adrain connected to a common node; a power NMOS transistor which has adrain connected to the common node and a source to which a second powersupply voltage is applied; an output node filter which is connected tothe common node; a phase lead-lag compensator which lead-lag compensatesfor the phase of an output signal of the output node filter; an erroramplifier which compares an output signal of the phase lead-lagcompensator with an input audio signal and amplifies an error betweenthe output signal and the input audio signal; a comparator whichconverts an error amplified by the error amplifier into a pulse signal;and a gate driver which controls a gate of the power PMOS transistor anda gate of the power NMOS transistor in response to the pulse signal,wherein the phase lead-lag compensator increases self-oscillationfrequency by lead-lag-compensating for the phase of the output signal ofthe output node filter.
 2. The digital audio amplifier of claim 1further comprising a bandwidth control means which adjusts the bandwidthof the error amplifier.
 3. The digital audio amplifier of claim 1,wherein the phase lead-lag compensator comprises: a first resistor whichis provided between an output node of the output node filter and anoutput node of the phase lead-lag compensator; a capacitor which isconnected to the first resistor in parallel between the output node ofthe output node filter and the output node of the phase lead-lagcompensator; and a second resistor which is provided between the outputnode of the phase lead-lag compensator and ground voltage.
 4. Thedigital audio amplifier of claim 1, wherein the phase lead-lagcompensator comprises: a first resistor which is provided between anoutput node of the output node filter and an inner node; a capacitorwhich is connected to the first resistor in parallel between the outputnode of the output node filter and the inner node; a second resistorwhich is provided between the inner node and an output node of the phaselead-lag compensator; and a third resistor which is provided between theoutput node of the phase lead-lag compensator and ground voltage.
 5. Thedigital audio amplifier of claim 1, wherein the phase lead-lagcompensator comprises: a first resistor which is provided between anoutput node of the output node filter and an output node of the phaselead-lag compensator; a second resistor which is provided between theoutput node of the phase lead-lag compensator and an inner node; a thirdresistor which is provided between the inner node and ground voltage;and a capacitor which is provided between the output node of the outputnode filter and the inner node.
 6. The digital audio amplifier of claim2, wherein the error amplifier comprises: an operational amplifier whichhas an output node connected to an input node of the comparator and afirst input node connected to the input audio signal; a first resistorwhich is provided between an output node of the phase lead-lagcompensator and a second input node of the operational amplifier; asecond resistor which is provided between the second input node of theoperational amplifier and the output node of the operational amplifier;and a capacitor which is connected to the second resistor in parallelbetween the second input node and output node of the operationalamplifier.
 7. The digital audio amplifier of claim 1, wherein the outputnode filter comprises: an inductor which is provided between the commonnode and an output node of the output node filter; a first capacitor oneend of which is connected to the output node of the output node filter;and a first resistor which is provided between the other end of thefirst capacitor and ground voltage.
 8. The digital audio amplifier ofclaim 7, wherein the phase lead-lag compensator comprises: a secondresistor which is provided between the output node of the output nodefilter and the output node of the phase lead-lag compensator; a secondcapacitor which is connected to the second resistor in parallel betweenthe output node of the output node filter and the output node of thephase lead-lag compensator; a third resistor which is provided betweenthe output node of the phase lead-lag compensator and the groundvoltage; and a third capacitor which is provided between the output nodeof the phase lead-lag compensator and the other end of the firstcapacitor.